The exemplary embodiment relates to the digital imaging arts. It finds particular application in conjunction with a method and apparatus for utilizing miniature security marks for watermarking documents and may be used to distinguish authentic documents from counterfeit documents.
Machine readable information in the form of watermarks, barcodes, and the like has been embedded into images on paper for a variety of applications, such as document identification and authenticity verification. The code is generally invisible or visually unobstructive and may be decoded by a device which is capable of reading the information. Current counterfeit prevention systems are frequently based on the use of digital watermarks. Digital watermarking is a technique which allows a user to add information (e.g., copyright notices, security codes, identification data, etc.) to digital image signals and documents. Such data can be in a group of bits describing information pertaining to the signal or to the author of the signal (e.g., name, place, etc.). Most common watermarking methods for images work in spatial or frequency domains.
For spatial digital watermarking, the simplest method involves flipping the lowest-order bit of chosen pixels in a gray scale or color image. This tends to work well only if the image will not be subject to any human or noisy modification. A more robust watermark can be embedded in an image in the same way that a watermark is added to paper. Such techniques may superimpose a watermark symbol over an area of the picture and then add some fixed intensity value for the watermark to the varied pixel values of the image. The resulting watermark may be visible or invisible depending upon the value (large or small, respectively) of the watermark intensity.
Spatial watermarking can also be applied using color separation. In this way, the watermark appears in only one of the color separations. This renders the watermark visibly subtle such that it is difficult to detect under regular viewing. There are several drawbacks to utilizing digital watermarking technology. To retrieve a watermark, extraction hardware and/or software is generally employed. As the digital watermarks usually have fairly large footprints, detectors employed to read the digital watermarks often require significant buffering storage. Consequently, this increases the detection costs, particularly if the watermark extraction is implemented in hardware.
There remains a need for alternative systems and methods to provide watermarking techniques for identification of images and/or documents, for uses such as prevention of counterfeiting.